Synchronous Stability Analysis and Enhancement of Solar-Hydro Integrated System via VSC-HVDC Transmission

This paper studies the synchronization stability of a bundled power export system where a large-scale solar plant and a hydropower station are interconnected and delivered to the receiving grid through a VSC-HVDC link. Under several simplifying assumptions, the system is modeled for both normal operation and a severe three-phase fault, with the converter current-limiting behavior explicitly included because it reshapes the fault-time power transfer and directly impacts the rotor-angle dynamics of the hydropower synchronous generator. Using Thevenin's theorem, a fault-time equivalent model is reconstructed to capture the interaction among the sending-end converter, AC network, and generator under current saturation, and the saturated-current control is parameterized by a current saturation angle that determines the active/reactive current allocation during faults. Based on the resulting simplified model, the optimal saturation angle is obtained by formulating a constrained optimization problem and solving it via the Lagrange multiplier method in combination with the equal-area criterion, aiming to maximize the transient synchronizing margin and avoid loss of synchronism. Numerical simulations validate that the derived saturation angle provides superior synchronization-stability performance compared with conventional current-limiting settings, thereby enhancing fault ride-through capability for hydro-PV bundled VSC-HVDC transmission systems.